Pilot allocation in multi-carrier systems with frequency notching

ABSTRACT

The present invention relates to a transmitting apparatus ( 62 ) for transmitting signals in a multi carrier system, in which pilot signals and data mapped on frequency carriers are transmitted in a transmission bandwidth, wherein a part of said transmission bandwidth is not used to transmit signals, comprising 
     pilot signal mapping means ( 63 ) for mapping pilot signals onto selected frequency carriers according to a pilot pattern which is adapted for a channel estimation in a corresponding receiving apparatus, said pilot pattern enabling a channel estimation for frequency carriers next to said part of said transmission bandwidth which is not used to transmit signals. The present invention further relates to a corresponding method as well as a pilot pattern.

The present invention relates to an improved pilot signal allocationscheme in a multi-carrier system, in which a part of the transmissionbandwidth is not used to transmit signals.

In the modern communication world, different kinds of communicationsystems often share the same frequency range or use overlappingfrequency ranges while operating in the same or neighbouring areas. Forexample, terrestrial services and cable based communication systemsoften use similar frequency ranges. Hereby, radiation from the cablenetwork, for example from unshielded parts of cable outlets and/orconnections to TV sets may disturb the operation of the terrestrialservices. On the other hand, the transmission quality of the cableservices may be negatively influenced by the terrestrial servicescausing additional noise in the cable medium. Particularly in situationswhere the terrestrial services are security relevant (emergencyservices, airport control and the like), it is necessary to takecorresponding counter measures in the cable communication system. Inorder to avoid such frequency range conflicts, a notching of frequenciesis often used in order to avoid negative effects in one or both of thecommunication systems or services.

FIG. 1 shows a frequency/amplitude diagram with an example of a spectrum1 of a terrestrial wireless short wave radio service transmittingsignals in five relatively narrow frequency bands 1′, as well as aspectrum 2 which is notched at the narrow frequency bands 2′ in whichthe terrestrial services are transmitting. Non-limiting examples forsuch terrestrial services are radio amateur transmissions, short waveradio services, security related radio transmissions, for example flightsecurity, and many more. Non-limiting examples for transmission orcommunication systems which are or need to be notched are cablebroadcast transmissions, powerline communication systems, xDSL systemsand many more.

It is to be understood, however, that the concept of notching can begenerally used in any wireless or wired transmission or communicationsystem which overlaps with the frequency range of any other wireless orwired transmission or communication system which operates in a smallerbandwidth. It has also to be understood that the concept of notchingsmall frequency ranges within a wider frequency range can be applied andused in unicast, multicast and broadcast transmission systems alike, aswell as in any kind of wired or wireless communication system. In orderto maximize the transmission or communication capacity, the width of thefrequency notches should be as small as possible, which means that onlythe frequencies which really overlap with the frequency that should beomitted and not used.

The notching of frequencies, particularly in situations where thenecessary notch width varies sometimes or regularly, has, however,negative impacts on the channel estimation, since important parts, suchas pilot signals, necessary for the channel estimation in the frequencyareas (frequency carriers) adjacent to the notched frequencies are lost.

The present invention therefore has the object to enable a more reliablechannel estimation in multi-carrier systems, in which pilot symbols areused for the channel estimation and in which a part of the frequencybandwidth is not used to transmit signals.

The above object is achieved by a transmitting apparatus according toclaim 1, a transmitting method according to claim 10 and a pilot patternaccording to claim 11.

According to the present invention, a transmitting apparatus fortransmitting signals in a multi-carrier system, in which pilot signalsand data mapped on frequency carriers transmitted in a transmissionbandwidth, wherein a part of said transmission bandwidth is not used totransmit signals, comprises pilot signal mapping means for mapping pilotsignals onto selected frequency carriers according to a pilot patternwhich is adapted for a channel estimation, said pilot pattern enabling achannel estimation for frequency carriers next to said part of saidtransmission bandwidth which is not used to transmit signals.

According to the present invention, a transmitting method fortransmitting signals in a multi-carrier system, in which pilot signalsand data mapped on frequency carriers are transmitted in a transmissionbandwidth, wherein a part of said transmission bandwidth is not used totransmit signals, comprises the steps of mapping pilot signals ontoselected frequency carriers according to a pilot pattern which isadapted for a channel estimation on a receiving side, said pilot patternenabling a channel estimation for frequency carriers next to said partof said transmission bandwidth which is not used to transmit signals.

The present invention is further directed to a pilot pattern for amulti-carrier system, in which pilot signals and data mapped onfrequency carriers are transmitted in a transmission bandwidth, whereina part of said transmission bandwidth is not used to transmit signals,said pilot pattern comprising pilot signals mapped onto selectedfrequency carriers according so that a channel estimation for frequencycarriers next to said part of said transmission bandwidth which is notused to transmit signals is possible.

The above object is further achieved by a receiving apparatus, areceiving method, a system, and a further method.

According to the present invention, a receiving apparatus for receivingsignals in a multi-carrier system, in which pilot signals and datamapped on frequency carriers transmitted in a transmission bandwidth,wherein a part of said transmission bandwidth is not used to transmitsignals, comprises channel estimation means for performing a channelestimation for received signals on the basis of pilot signals arrangedin a pilot pattern, said pilot pattern enabling a channel estimation forfrequency carriers next to said part of said transmission bandwidthwhich is not used to transmit signals.

According to the present invention, a receiving method for receivingsignals in a multi-carrier system, in which pilot signals and datamapped on frequency carriers transmitted in a transmission bandwidth,wherein a part of said transmission bandwidth is not used to transmitsignals, comprises the step of performing a channel estimation forreceived signals on the basis of pilot signals arranged in a pilotpattern, said pilot pattern enabling a channel estimation for frequencycarriers next to said part of said transmission bandwidth which is notused to transmit signals.

The present invention is further directed to a system comprising atransmitting apparatus for transmitting signals in a multi carriersystem, in which pilot signals and data mapped on frequency carriers aretransmitted in a transmission bandwidth, wherein a part of saidtransmission bandwidth is not used to transmit signals, comprising pilotsignal mapping means for mapping pilot signals onto selected frequencycarriers according to a pilot pattern which is adapted for a channelestimation in a corresponding receiving apparatus, said pilot patternenabling a channel estimation for frequency carriers next to said partof said transmission bandwidth which is not used to transmit signals,said system further comprising a receiving apparatus according to thepresent invention adapted to receive signals transmitted from saidtransmitting apparatus.

The present invention is further directed to a method for transmittingand receiving signals in a multi carrier system, in which pilot signalsand data mapped on frequency carriers are transmitted in a transmissionbandwidth, wherein a part of said transmission bandwidth is not used totransmit signals, comprising the steps of mapping pilot signals ontoselected frequency carriers according to a pilot pattern which isadapted for a channel estimation on a receiving side, said pilot patternenabling a channel estimation for frequency carriers next to said partof said transmission bandwidth, transmitting said signals, and receivingsaid signals according to the receiving method of the present invention.

In order to enable a more reliable channel estimation (or a channelestimation with increased reliability) on a receiving side, e.g. areceiving apparatus, in multi-carrier systems, in which a part (orseveral parts) of the transmission bandwidth is not used to transmitsignals, i.e. in which one or more parts of the frequency bandwidth arenotched, the present invention suggests to use a frequency (andeventually time) distribution of pilot signals, i.e. a pilot patternthat enables a channel estimation particularly for the frequencycarriers in the areas or regions next or adjacent to the part of thetransmission bandwidth which is not used to transmit signals. Thus, thepresent invention hereby suggests to use a pilot pattern in which nofrequency carriers with pilot signals are present in the part of saidtransmission band which is not used to transmit signals. Hereby, allpilot signals necessary for a more reliable channel estimation can bereceived and processed at the receiving side.

The present invention can generally be applied to any wireless or wiredunidirectional (point-to-point), multidirectional or broadcasttransmission system, communication system or transmission link in whichmultiple frequency carriers are used to transmit data, pilot signals andother necessary information. An non-limiting example for such a systemis an orthogonal frequency division multiplexing (OFDM) system, but thepresent invention can be applied in any system in which the transmissionor communication (frequency) bandwidth is split in individual frequencycarriers, onto which data, pilot signals (and other necessaryinformation) are mapped or modulated. Hereby, the frequency carriers maybe equidistant and all have the same length (bandwidth), such as in anOFDM system, or they may be not equidistant and/or not have all the samebandwidth.

Hereby, the present invention can be applied to systems in which pilotsignals are mapped onto frequency carriers among the data carryingfrequency carriers (a non-limiting example of such a system would be aclassical OFDM broadcasting system, such as a digital video broadcastsystem in which only pilot carriers embedded in the data carriers areused for channel estimation on the receiving side) or to systems inwhich a preamble with pilot signals is used for synchronization andchannel estimation on a receiving side and in which the data carriersare transmitted in separate data symbols (an example for such a systemwould be a typical bi-directional communication system like powerlinecommunication systems, or newer broadcast standards which use a preamblewith pilot signals to define the start of a new data symbol frame). Apossible application (among others) would be a next generation or futurecable-based digital video broadcast system. However, the presentinvention is not limited to these examples but can be applied to othersystems, such as systems which use a mixture of preambles with pilotsignals and data symbols with embedded pilot signals.

In a multi-carrier system in which frequency carriers with pilot signalsare embedded within frequency carriers with data, i.e. data and pilotsignals are mixed, the present invention suggests to use (and to performa channel estimation on the basis of) a pilot pattern with additionalpilot signals on frequency carriers adjacent said part of saidtransmission bandwidth which is not used to transmit signals. In otherwords, the original distribution of and allocation of pilot signals,i.e. the pilot pattern, outside of the frequency notch remainsunchanged, except that additional pilot signals are mapped onto some orall frequency carriers adjacent to the part of the transmissionbandwidth which is not used to transmit signals. The term adjacent to,hereby means throughout this specification that the additional pilotsignals should be added to carriers between the notched frequency partand the next carriers of the original (unchanged) pilot pattern. Forexample, the additional pilot signal could be added only to carriersimmediately adjacent to or bordering the frequency notch, but alsoadditionally or alternatively to other carriers close to but notimmediately adjacent the notch.

These additional pilot signals can then be used at the receiving sidefor the channel estimation of frequency carriers in the areas adjacentto the part of the transmission bandwidth which is not used to transmitsignals. Hereby, in case that the pilot pattern has a distribution ofpilot signals in the time dimension, the additional pilot signals couldbe mapped on frequency carriers adjacent to said part of saidtransmission bandwidth which is not used to transmit signals with a timedistribution which corresponds to the time distribution of the originalpilot pattern and the channel estimation can be performed on the basisof such a pilot pattern. The time distribution could be regular or notregular. In other words, in a system in which the frequency carriers areadditionally allocated to time slots, such as in an OFDM system, itmight not be necessary to allocate additional pilot signals to everyfrequency carrier and every time slot adjacent to the part of thetransmission bandwidth which is not used to transmit signals, but itmight be sufficient to allocate the additional pilot signals tofrequency carriers adjacent to said part of the transmission bandwidthwhich is not used to transmit signals in a time slot distribution whichcorresponds to the time slot distribution of the original pilot patternand to perform the channel estimation on the basis of such a pilotpattern. Hereby, an increased capacity for data transmission and a stillmore reliable channel estimation is achieved. Alternatively, additionalpilot signals may be mapped onto every frequency carrier adjacent tosaid part of said transmission bandwidth which is not used to transmitsignals in the time dimension and the channel estimation could beperformed on the basis of such a pilot pattern with additional pilotsignals. Hereby, a more reliable channel estimation on a receiving sidecould be achieved.

The present invention is further applicable to multi-carrier systems inwhich frequency carriers with pilot signals are arranged in one or moretraining or preamble patterns and frequency carriers with data arearranged in one or more data patterns. In other words, the pilot signalsare transmitted in preamble symbols or training symbols and the data aretransmitted in data symbols, whereby the preamble symbols or trainingsymbols are used on the receiving side for channel estimation (andeventually for additional tasks such as frequency and timesynchronization and so forth depending on the system requirements). Insuch systems, the present invention suggests to map additional pilotsignals on frequency carriers for said data pattern adjacent to the partof the transmission bandwidth which is not used to transmit signals andto perform the channel estimation on the basis of such a pilot pattern.Since a part of the frequency carriers with pilot signals of thetraining or preamble pattern can not be transmitted due to the frequencynotching, the present invention suggests to arrange and map pilotsignals onto frequency carriers of the data pattern adjacent to thefrequency notch and to perform the channel estimation on the basis ofsuch a pilot pattern. In other words, since the training pattern(usually) only comprises pilot signals arranged in a certain way, thepresent invention suggests to map additional pilot symbols not withinthe training pattern, but within the data pattern in order to enable amore reliable channel estimation on the receiving side. Hereby, thepresent invention suggests to either map additional pilot signals onevery carrier for the data pattern adjacent to said frequency notch, orto map the additional pilot signals with a (regular or not regular) timedistribution corresponding to the (regular or not regular) timedistribution of the original pilot pattern and to perform the channelestimation on the basis of the resulting pilot pattern.

It is to be understood that the two above explained concepts of thepresent invention could be mixed and applied to systems in which pilotsignals are transmitted in preambles or training patterns as well asembedded within data in data symbols.

Further, it should be understood that the above explained concepts forpilot patterns for multi-carrier systems with notched frequencies can beused in permanent or semi-permanent systems, in which the notchedfrequencies are known at the time when the system is being set up, sothat the pilot pattern can be adapted from the beginning to the notchedfrequencies and the transmitting side (apparatus) as well as thereceiving side (apparatus) know about the presence, the location and thewidth of the notched frequencies. Hereby, no further informationexchange between transmitter and receiver in relation to the notchedfrequencies is necessary during the operation of the system and thesystem setup (including eventual updates from time to time) is doneexternally. Alternatively, the above explained concepts for pilotpatterns for multi-carrier systems with notched frequencies can be usedin dynamic systems, in which it is necessary to adjust and change thelocation and the width of the notched frequencies regularly or from timeto time. This can for example be implemented by enabling the transmitterto dynamically change the pilot pattern depending on the presence of afrequency notch and by enabling the receiver to perform said channelestimation on the basis of such a dynamically changed pilot pattern. Thepresence (and location and width etc.) of the frequency notch could bedetected by the transmitter and signalled to the receiver (or viceversa) or could be detected by another entity of the system andsignalled the both the transmitter and the receiver. Hereby, thesignalled information could include only the location (and width etc.)of a frequency notch and the transmitter and the receiver then wouldknow which new pilot pattern shall be used due to pre-stored informationin this respect. Or, the signalling information would include thelocation (and width etc.) of the notch as well the respective new pilotpattern to be used.

As an alternative or as an addition to the use of additional pilotsignals on frequency carriers adjacent to the part of the transmissionbandwidth which is not used to transmit signals, the present inventionsuggests to shift the original pilot pattern in the frequency dimensionand to perform the channel estimation on the basis of such a shiftedpilot pattern so that a channel estimation for frequency carriers nextto said part of said transmission bandwidth which is not used totransmit signals is possible. In cases where the frequency notch, i.e.the part of the transmission bandwidth which is not used to transmitsignals, is smaller than the (time interpolated) repetition rate of thepilot signals in the frequency direction, it is possible, if some pilotsignals of the original pilot pattern fall within the frequency notch,to shift the pilot pattern in the frequency dimension so that thefrequency notch falls within two adjacent frequency signals in thefrequency dimension. Thereby, no pilot signals are lost and thereceiving side is able to perform a more reliable channel estimation forall received frequency carriers. On the other hand, in case that thefrequency notch is larger than the distance between two adjacent pilotsignals in the frequency dimension, additional pilot signals have to bemapped onto the frequency carriers immediately adjacent the frequencynotch as explained above.

It is to be understood that the terms transmitting apparatus andreceiving apparatus used in the present specification are intended tocover all possible present and future wireless, wired, mobile, portable,non-portable, stand-alone, combined and the like devices, and areintended to include all possible implementations of the described andclaimed functionalities. For example the transmitting apparatus caninclude receiving functionalities including but not limited to thefunctionalities described for the receiving apparatus of the presentinvention and vice versa. Further, it is to be noted that the termpattern, as e.g. used for pilot, preamble, training or data pattern isintended to describe the situation in the frequency domain in which thebaseband processing of devices of frequency multi-carrier systemnormally takes place, including the modulation or mapping of pilotsignals, data signals or other information signals onto the frequencycarriers. The term symbol is then to describe the situation in the timedomain into which the frequency domain signals (or patterns) weretransformed in the transmitter and then transmitted after therespectively necessary processing depending on the used transmission orcommunication system. Also, it should be understood that the presentinvention is intended to cover all present and future frequency rangesfor the transmission bandwidth used in the multi-carrier system. Also,the present invention is intended to cover all possible locations,widths, etc. of the frequency notch or notches in the transmissionbandwidth. The present invention is also not limited to any specifickind, width etc. of frequency carriers.

The present invention is described in more detail in the followingdescription of preferred embodiments in relation to the encloseddrawings, in which

FIG. 1 shows a frequency/amplitude diagram of a wideband frequencyspectrum which is notched due to several small band services,

FIG. 2 shows a frequency/time diagram of data carriers with embeddedpilot carriers,

FIG. 3 shows a frequency/time diagram of FIG. 2 for the case that a partof the transmission bandwidth is not used for transmission,

FIG. 4 shows the frequency/time diagram of FIG. 3 with additional pilotsignals,

FIG. 5A shows a frequency domain representation of a preamble and

FIG. 5B shows a time domain representation of the preamble of FIG. 5A,

FIG. 6A shows a further example of a frequency domain representation ofa preamble and

FIG. 6B shows a time domain representation of the preamble of FIG. 6A,

FIG. 7 shows a schematic representation of an example of a OFDM burstwith shortened training symbols and several data symbols,

FIG. 8 shows a frequency domain representation of a shortened preambleor training pattern in which a part of the transmission bandwidth is notused for transmission,

FIG. 9A shows again the shortened preamble of FIG. 8 and

FIG. 9B shows the frequency domain representation of a correspondingdata pattern with additional pilot signals at the edges of a part of thetransmission bandwidth which is not used for signal transmission,

FIG. 10 shows a frequency/time diagram similar to FIG. 2 but for thecase of two transmitters,

FIG. 11 shows a frequency/time diagram for the case of FIG. 10 withadditional pilot signals at the edges of the part of the transmissionbandwidth which is not used for signal transmission,

FIG. 12 shows a frequency/time diagram of pilot signals embedded in datasignals in which the distance between adjacent pilot signals in thefrequency dimension is larger than the width of the unused transmissionbandwidth,

FIG. 13 shows the case of FIG. 12 but with a shifted pilot pattern sothat the unused transmission bandwidth does not coincide with pilotsignals,

FIG. 14 shows a schematic block diagram of a transmitting apparatusaccording to the present invention, and

FIG. 15 shows a schematic block diagram of a receiving apparatusaccording to the present invention.

In the following description of preferred embodiments, the presentinvention is explained on the basis of an OFDM system, in which data,i.e. signalling data, information content data or any other kind ofdata, and pilot signals are mapped onto mutually orthogonal frequencysubcarriers. As set-out above, however, the present invention may beapplied to any communication system or transmission system or link usinga plurality of separate frequency carriers, onto which the data, thepilot signals and so forth are mapped, for the transmission within thegiven frequency bandwidth.

FIG. 2 shows a schematic frequency/time diagram of subcarriers of anOFDM system in which a plurality of pilot signals arranged in a pilotpattern are embedded within a (temporal) stream of data signals mappedonto respective frequency subcarriers. Most broadcast systems use acontinuous transmission of data symbols in which pilot signals areembedded, but some of the recently proposed broadcast systems use atemporal frame structure in which pilot symbols are transmitted intraining symbols or preamble symbols and the data are transmitted indata symbols. Bi-directional communication systems in which content,signalling data and so forth is exchanged between two transceiverstypically use temporal frame structures, such as frame bursts, but mayalso use other suitable structures.

FIG. 2 shows an example in which the first frequency subcarrier in eachtime slot carries a pilot signal 3, 4, 5, 6. Further, a pilot patternwith a regular frequency and time distribution of pilot signals 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17 is distributed over and embedded in thesubcarriers carrying data. Hereby, in the first time slot shown in thediagram of FIG. 2, the forth subcarrier carries a pilot signal 7, aswell as the thirteenth subcarrier 11 and the twenty-second subcarrier15. In the second time slot, the seventh subcarrier 9, the sixteenthsubcarrier 13 and the twenty-fifth subcarrier 17 carry pilot signals. Inthe third time slot, the tenth subcarrier 10 and the nineteenthsubcarrier 14 carry pilot signals. In the fourth time slot, the pilotsignals are arranged on the same subcarriers as in the first time slot,namely the fourth subcarrier 8 carries a pilot signal, the thirteenthsubcarrier 12 as well as the twenty-second subcarrier 16. In otherwords, the arrangement of the pilot signals is repeated every fourthtime slot, the pilot signals are arranged on every tenth time slot andthe shift between the pilot signals in a specific time slot and theimmediately succeeding time slot is three subcarriers. For example, thepilot signal on the fourth subcarrier 7 in the first time slot and thepilot signal on the seventh subcarrier in the second time slot in FIG. 2are shifted by three subcarriers.

It is to be understood that the repetition rate of the pilot signals ineach time slot as well as the temporal repetition rate of the pilotsignals as shown in FIG. 2 (as well as the other figures of thespecification) are only an example and that any other pilot pattern canbe used depending on the respective system requirements. Further,although regular pilot patterns with a repetitive and/or regularstructure seem to be more commonly used, it is possible to use any kindof suitable regular or irregular pilot pattern (in the time and/orfrequency dimension) depending on the system requirements. In thepresent description, the term pilot pattern is not restricted to anykind of regular or repetitive pattern but includes any kind of suitablearrangement of the pilot signals.

After the generation of a subcarrier pattern with pilot signals and datasignals as for example shown in FIG. 2 and the correspondingtransformation of the subcarrier pattern into the time domain and theprocessing and transformation of time domain signals into the actualtransmission signals, symbol bursts and the like depending on therespectively used communication or transmitting system in a respectivetransmitting apparatus or transceiving apparatus, the signals arereceived in a receiving or transceiving apparatus on a receiving sideand processed back into the frequency domain. In the receiving ortransceiving apparatus receiving the signals, the signals are processedand transformed back into the frequency domain. The received pilotsignals are then used to perform a channel estimation for the datacarrying frequency subcarriers the content and the characteristics ofthe pilot signals are known to the receiver, for example, the receiverknows the amplitude and the phase of the pilot signals which are to beexpected. Often, pseudo noise (pn) sequences are used but other suitablesequences can be applied. The comparison of the known and expected pilotsignal with the actually received pilot signal enables the receiver toperform channel estimation for the data carrying frequency subcarriersbetween adjacent pilot signals in the frequency as well as in the timedimension. For example, the receiver may first perform a timeinterpolation between two adjacent pilot signals, where after the timeinterpolated values for each frequency subcarrier are interpolated againin the frequency direction, so that a channel estimation and correctionvalue is obtained for each subcarrier. Of course, other ways to obtainthe channel estimation values for the data subcarriers can be used.

The specific choice of the pilot pattern and the pilot signal densitydepends on the system requirements and architecture. Although anincrease of the number of pilot signals normally enhances the channelestimation quality, the transmission capacity is decreased, so that thedesign of the pilot pattern is always a compromise between channelthroughput and channel estimation quality. An important design factor isthe so called Nyquist criterion, whereby often some oversampling isadded in order to guarantee a proper channel estimation on the receivingside.

However, most systems are designed in a way that the channel estimationquality is reduced if one or more pilot signals are missing or cannot bereceived on the receiving side. An example is visualized in FIG. 3 whichshows the frequency/time diagram of FIG. 2 with a part 18 of thetransmission bandwidth missing. In case of notched systems, as explainedabove, a small part of the entire transmission bandwidth is cut-out ornotched and therefore not used for the transmission of signals. In theexample of FIG. 3, a number of for example six subcarriers are not usedto transmit signals, so that the pilot signals of the subcarriers, inthe shown example the pilot signals 11, 12, 13 are not transmitted andcan therefore not be used to perform a channel estimation on a receivingside. Consequently, the subcarriers 19 and 20 in the areas adjacent tothe part 18 which is not used for signal transmission cannot be properlychannel estimated on the receiving side. These adjacent areas consist ofthe subcarriers between the frequency notch 18 and the next (infrequency dimension) subcarriers with pilot signal, in the shown examplethe subcarriers 10 and 14.

In a first embodiment, the present invention suggests to map additionalpilot signals to the frequency subcarriers adjacent the part 18 of thetransmission bandwidth which is not used for signal transmission, as forexample shown in FIG. 4. FIG. 4 shows the frequency/time diagram of FIG.3, whereby additional pilot signals 21, 22, 23, 24, 25, 26, 27, 28 aremapped onto every frequency subcarrier immediately adjacent or at theedge of the part 18 of the transmission bandwidth which is not used forsignal transmission. If an additional pilot signal is mapped to everyfrequency subcarrier immediately adjacent the part 18, the reliabilityof the channel estimation on the receiving side is significantlyenhanced. However, in order not to reduce the data throughput too much,it can be sufficient for a more reliable channel estimation ifadditional pilot signals are mapped not on every frequency subcarrierimmediately adjacent the part 18, but only onto some of the frequencysubcarriers. For example, it might be sufficient if the additional pilotsignals have the same time distribution as the original pilot pattern.In the example of FIG. 4, it might thus be sufficient if only additionalpilot signals 21, 24, 25, 28 are mapped onto the frequency subcarriersimmediately adjacent the part 18, so that the temporal repetition rateof three symbols of the original pilot pattern is maintained. Thefrequency subcarriers 22, 23, 26, 27 would then be available for datasignals. More generally, it might be sufficient to map additional pilotsignals on subcarriers adjacent the part 18 in any kind of suitablefrequency and/or time scheme or distribution which enables a good andmore reliable channel estimation on the receiving side but stillprovides a good data throughput. In such a case, the density ofadditional pilot signals might even be wider or less wide than the timeand/or frequency distribution of the original pilot pattern. Also, itmight be advantageous to allocate the additional pilot signals only tosubcarriers immediately adjacent to the frequency notch 18, but it isalso alternatively or additionally possible to add the additional pilotsignals to any suitable subcarriers between the frequency notch 18 andthe next (in frequency dimension) subcarriers with pilot signal, in theshown example the subcarriers 10 and 14. also, it might be advantageousto use pseudo noise (pn) sequences for the additional pilot signals, butother suitable sequences can be applied. The above statement in relationto the location, nature and characteristics of the additional pilotsignals apply to all embodiments described herein.

The above example and embodiment of the invention was based on a systemin which pilot signals are embedded within data carriers, such as forexample in classical broadcast systems in which the data are transmittedin a continual temporal stream with the pilots embedded therein.

The following example and embodiment is directed to systems in whichpilot signals are transmitted in preamble symbols or training symbolsand the data are transmitted in data symbols. In bi-directionalcommunication systems, the preambles or training symbols are for exampleused for time and/or frequency synchronization, (frequency and samplingfrequency) off-set correction, channel estimation and/or automatic gaincontrol adjustment on the receiving side. In more recently proposedbroadcast OFDM systems, preambles or training symbols are for exampleused at the start of each temporal frame, wherein a frame comprises oneor more preambles or training symbols and multiple data symbols, forinitial channel estimation and/or off-set correction and also for thepotential signalling of very basic transmission parameters.

FIG. 5A shows a frequency domain representation of an example ofpreamble or training pattern in which every subcarrier 29 carries apilot signal, so that for example all available 2048 frequencysubcarriers carry a respective pilot. FIG. 5B shows a time domainrepresentation of the preamble or training pattern of FIG. 5A. The timedomain preamble symbol or training symbol has in the shown example 2048time samples 30 forming the symbol and the repetition rate is 2048 timesamples. FIG. 6A shows a frequency domain representation of a preamblepattern or training pattern in which only every fourth subcarrier 31carries a pilot signal and the intermediate subcarriers 32 are mappedwith zeros. After transformation in the time domain, as shown in FIG.6B, the time domain signal of the preamble symbol or training symbolshows four repetitions (corresponding to the fact that every fourthsubcarrier carries a pilot signal), each repetition pattern 33, 34, 35,36 having the identical time samples 37. Every repetition pattern has alength of 512 samples so that the overall symbol length is again 2048samples in the shown example. Of course, other numbers may beappropriate depending on the wanted application and the usedcommunication system. Generally, a decreased pilot signal density in thefrequency domain results in a higher number of repetitions in the timedomain. These shortened training symbols or preamble symbols, i.e. eachrepetition pattern is considered to be a shortened training symbol orpreamble symbol, still enables a complete channel estimation if theusual conditions, such as the Nyquist conditions are fulfilled. FIG. 7shows a schematic example of a typical OFDM burst with shortenedtraining symbols 38 followed by a number of data symbols 39. Theshortened training symbols enable a reliable and good channel estimationin a shorter time as compared to longer training symbols. In ourexamples, the shortened training symbols resulting from the example ofFIG. 6 enable a much faster but still reliable channel estimation ascompared to the example of FIG. 5.

However, in case that a part of the entire transmission bandwidth is notused for signal transmission, this means that a part of the preamblesymbol or training symbol will not be transmitted so that the receivingside lacks pilot symbols for the channel estimation. An example is shownin FIG. 8 which shows a frequency domain representation of a preamblepattern or training pattern similar to the one of FIG. 6A, whereby apart 40 of the entire transmission bandwidth is not used for signaltransmission. Particularly in the case where only every m-th frequencysubcarrier of the training or preamble pattern (m being a natural numberlarger 1) carries a pilot signal, it is not possible to add additionalpilot signals on the frequency subcarriers immediately adjacent the part40 which is not used for signal transmission, for example thesubcarriers 41, since the repetition rate of the pilot signals in thepreamble pattern or training pattern would be disturbed and it would notbe possible to obtain proper repetition patterns in the time domain (cf.explanation of FIG. 6B). In this embodiment, the present inventiontherefore suggests to add additional pilot signals on the frequencysubcarriers of the data pattern which are immediately adjacent the part40 which is not used for signal transmission. FIG. 9A and FIG. 9Bvisualize this concept. FIG. 9A essentially corresponds to FIG. 8 andshows a frequency domain representation of the preamble or trainingpattern in which every fourth subcarrier 31 carries a pilot signal andthe intermediate subcarriers 32 are empty or carry zeros. The part 40 ofthe bandwidth which is not used to transmit signals would have carried apilot signal which can now not be transmitted to the receiver.Therefore, an additional pilot signal is added to each subcarrier 42immediately adjacent or on the edge of the part 40 which is not used forsignal transmission in the data pattern as shown in FIG. 9B. The datapattern normally carries only data signals on each frequency subcarrierbut now carries additional pilot signals on the subcarriers 42 adjacentto the frequency notch. Thus, on the receiving side, the subcarrierscarrying the data are channel estimated on the basis of the pilotsignals of the preamble or training pattern, except the subcarriers inthe regions adjacent to the part 40 of the unused transmissionbandwidth. In the example of FIG. 9B, the subcarriers 44 of the datapattern are for example channel estimated by using the additional pilotsignal 42 on the edge of the frequency notch close to them as well asthe pilot signal in the preamble or training pattern on the samesubcarrier as one of the data subcarriers. Similarly, the datasubcarriers 45 in the opposite region adjacent to the frequency notchare channel estimated by using the additional pilot signal 42 on theedge of the frequency notch and a pilot signal from the preamble ortraining pattern. Although FIG. 9B only shows a single data pattern, itshould be clear that depending on the design of the time domain OFDMbursts or symbols, it is possible that each burst comprises one or moredata symbols. In this case, it is possible to add the additional pilotsignals only in one data pattern, several data patterns or all datapatterns that follow the preamble or training pattern. All subcarriersin the data patterns that are not affected by the frequency notch canstill be channel estimated from the pilot signals in the shortenedtraining pattern. Subcarriers of the data pattern, which are affected bythe frequency notch can be channel estimated from a mix of pilot signalsfrom the training pattern and the additional pilot signal 42 of the oneor more data patterns. With such a concept, the time structure of theOFDM bursts remains unchanged.

It should be noted that in some systems a mixture between preamble ortraining patterns with pilot signals and data patterns with embeddedpilot signals are possible. For these cases it is possible to combinethe concepts of the present invention as explained in relation to FIG. 4and FIG. 9B.

The concept of the first and the second embodiment is also applicable toMIMO (multiple input multiple output) systems with two or moretransmitters. Such MIMO systems need orthogonal pilot structures inorder to enable the receiver or the receivers to extract the channelestimates of all available propagation paths. For MIMO systems with two(or more) transmitters, alternating pilot structures can be used,whereby one transmitter transmits the original pilot pattern and theother transmitter alternates the sign (or uses complex conjugates) ofthe original pilot structure for every other pilot in frequency (andeventually also in the time) direction. It is also possible to usespecific orthogonal pilot signals (without inversion) transmitted by thetwo or more transmitters as long as a receiver can distinguish fromwhich transmitters the respective pilot signals come.

FIG. 10 shows a frequency time diagram of (a non-limiting example of) aMIMO system with two transmitters similar to the one shown in FIG. 2.Hereby, in MIMO systems, the first two subcarriers of each time slot andthe last two frequency subcarriers in each time slot are mapped with arespective pilot signal. In FIG. 10, in addition to the pilot signals 3,4, 5, 6 of the first frequency subcarriers, the second subcarriers alsocarry pilot signals 3′, 4′, 5′ and 6′. Hereby, if the first transmittertransmits a time domain signal corresponding to the frequency domainsignal as shown in FIG. 10, the second transmitter will transmit asimilar signal, but with every other pilot signal having an invertedsign or being the conjugate complex, for example, the pilot signals 3′,4′, 5′ and 6′ transmitted by a second transmitter may be inverted(opposite sign or complex conjugate in relation to the respective pilotsignals transmitted by the first transmitter). Also, for example, thesecond transmitter may transmit the pilot signals 7, 9, 10, 12, 15, 17and so forth with inverted values as compared to the respective pilotsignals transmitted by the first transmitter. FIG. 11 shows theallocation of additional pilot signals in case that the system of FIG.10 is frequency notched. Similar to the previous embodiment especiallythe one described in relation to FIG. 4, additional pilot signals aremapped onto the frequency subcarriers immediately adjacent the part 18of the transmission bandwidth which is not used for signal transmission.Hereby, for the described MIMO system with two transmitters, the twoimmediately adjacent frequency subcarriers carry additional pilotsignals 21, 21′, 22, 22′, 23, 23′, 24, 24′, 25, 25′, 26, 26′, 27, 27′,28, 28′. Hereby, every other pilot signal transmitted by the secondtransmitter is inverted (sign alternation or complex conjugate) inrelation to the respective pilot signal in the same frequency subcarriertransmitted by the first transmitter. For example, the secondtransmitter may transmit the pilot signal 21, 22, 23, 24, 25, 26, 27, 28with the identical values as the first transmitter, but the pilotsignals 21′, 22′, 23′, 24′, 25′, 26′, 27′ and 28′ with respectivelyinverted values in relation to the first transmitter. The other pilotsignals, i.e. the original pilot pattern transmitted by the first andthe second transmitter accept in the frequency subcarriers within thefrequency notch remains unchanged.

In the following, a further alternative embodiment of the presentinvention is explained. FIG. 12 shows a frequency time diagram of asystem similar to the one explained in relation to FIG. 2, in which apilot pattern of pilot symbols is inserted or embedded within a streamof data carriers. Similar to FIG. 2, the system of FIG. 12 has a pilotsignal 50, 51, 52 and 53 in the first frequency subcarrier of each timeslot. Further, the system has a regularly distributed pattern of pilotsignals 54, 55, 56, 57, 58, 59, 60, 61 embedded in between frequencysubcarriers with data. A part 18 of the frequency bandwidth is not usedfor a transmission of signals, i.e. notched, so that for example pilotsignals 58 and 59 are not transmitted. However, since the width of thepart 18 which is not used to transmit signals is below the timeinterpolated repetition rate or distance between pilot signals and thefrequency direction, it is possible to just shift the entire pilotpattern (but not the pilots in the first frequency subcarriers) so thatthe part 18 is in between pilot signals in order to avoid that pilotsignals of the original pilot pattern fall within the frequency notch.FIG. 13 visualizes the situation after this shifting of the pilotpattern. Pilot pattern of FIG. 12 has been shifted by three subcarriersin the frequency dimension so that the frequency notch now lies inbetween adjacent pilots and no pilots fall within the part 18 which isnot used for signal transmission. Thus, the entire pilot pattern ispreserved, the receiving side can perform a good and reliable channelestimation for all frequency subcarriers. In other words, the pilotpattern is shifted by one or more frequency subcarrier positions inorder to have no pilots within the frequency notch. The shift, i.e. thenumber of subcarriers by which the pilot pattern is to be shifted, canbe signalled from a transmitter to a receiver as a basic physical layerinformation, for example part of a preamble, or by any other suitableway.

FIG. 14 shows a schematic diagram of a transmitting apparatus of thepresent invention, which comprises the necessary elements and structuresto perform a change of the pilot pattern as suggested by the presentinvention and described in the foregoing embodiments. It is to beunderstood that FIG. 14 (as well as FIG. 15) only show the structuralelements necessary and adapted to perform the functionalities of thepresent invention, but that additional elements which are necessary forthe actual operation of the transmitting and the receiving apparatus arenot shown for the sake of clarity. Correspondingly, the transmittingapparatus 62 of the present invention comprises a modulation means orelement 63 which is adapted to modulate or map pilot signals ontofrequency carriers of the respectively used multicarrier system. Thetransmitting apparatus 62 further comprises a pilot pattern informationmeans or element 64 which is adapted to provide the modulation means 63with information about how the pilot signals should be mapped onto thefrequency carriers, for example, if a part of the transmission bandwidthis to be notched, i.e. not to be used for signal transmission, andadditional pilot carriers should be allocated to subcarriers immediatelyadjacent the frequency notch, such as explained in relation to FIGS. 2to 11, or if the pilot pattern should be shifted as explained inrelation to FIGS. 12 and 13. The pilot pattern information means 64obtains such information and controls (or at least informs) themodulation means 63 correspondingly, so that additional pilot signalsare allocated or the entire pilot pattern is shifted. The pilot patterninformation means 64 can hereby receive the respective pilot patternchange and information from another entity via signalling information,or such information may be stored in the pilot pattern information means64 when the transmitting apparatus 62 of the present invention isinitialized, or the transmitting apparatus 62 has some kind ofpossibility to detect or measure frequency notches and to adapt thepilot pattern correspondingly. In such cases, a detector for frequencynotches can be included in the pilot pattern information means 64. Thetransmission apparatus 62 further comprises a modulation means 65 whichis adapted to modulate data signals onto frequency carriers on the basisof corresponding information from the pilot pattern information means64. Especially after changes of the pilot pattern, the modulation means65 needs to know which frequency carriers are available for data andwhich are not. The data carriers from the modulation means 65 and thepilot carriers from the modulation means 63 are then combined in linewith the required communication system, for example by embedding thepilot carriers within the data carriers, or by forming preamble ortraining patterns with the pilot signals and by forming separate datapatterns with the data signals. A signal forming means or element 66then forms a typical time domain signal or symbol from the pilot signalsand data signals after their transformation to the time domain (notshown), whereafter the just formed signals are processed and preparedfor a transmission by a transmission means 67 and transmitted by atransmission interface 68. The transmission interface 68 can be wirelessinterface, such as an antenna, an antenna pattern or the like, or awired interface.

FIG. 15 shows a schematic block diagram of a receiving apparatus 69according to the present invention, which is adapted to receive signalsfor example from a transmitting apparatus 62 of the present inventionand comprises the necessary structural elements adapted to perform thefunctionalities of the present invention as described in the aboveembodiments. Hereby, signals are received by a receiving interface 70,which may be a wireless interface, such as an antenna, an antennapattern or the like, or a wired interface. The received signals are thenprocessed, for example down converted or the like, by a receiving meansor element 71 and then demodulated in a demodulation means or element72. The demodulation means 72 performs a transformation of the received(time domain) signals into the frequency domain, i e. into frequencycarriers. The data carriers are then further processed, e.g. byde-mapping the data information from the frequency carriers, in a dataprocessing means or element 75. The pilot carriers which are embedded inthe data carriers or are present as separate training or preamblepatterns in the frequency carriers from the demodulation means 72 aredetected and processed by a channel estimation means or element 73 whichprovides the data processing means or element 75 with the necessaryinformation enabling a proper channel estimation and de-mapping of thedata carriers. Hereby, the receiving apparatus 69 comprising pilotpattern information means or element 74 provides the channel estimationmeans or element 73 with the necessary information about where and howthe pilot signals are mapped onto the frequency carriers. The pilotpattern information means or element 74 obtains or provides informationto the channel estimation means or element 73 about the presently usedor changed pilot patterns as described in the previous embodiments. Forexample, in case that additional pilot signals are added to frequencycarriers immediately adjacent a frequency notch as is explained inrelation to FIGS. 2 to 11, pilot pattern information means or element 74provides such information to the channel estimation means 73 so that aproper channel estimation is enabled. Similarly, in case that the pilotpattern is shifted as described in relation to FIGS. 12 and 13, thepilot pattern information means or element 74 provides such informationto the channel estimation means or element 73. Information about shiftedpilot patterns or additional pilot signals can be obtained in thereceiving apparatus 69 from a transmitting apparatus by means ofsignalling data or the like, or through another channel informing thereceiving apparatus 69 about changes in the pilots and/or the pilotpattern. Also, the receiving apparatus may just receive informationabout notched frequencies or frequency bands, whereafter the pilotpattern information means or element 74 automatically knows which pilotpattern changes or additional pilot signals will be used and informs thechannel estimation means 73 correspondingly.

It is to be understood that every additional pilot signal result in thedecreased data throughput. Therefore, in order to save overhead, adynamic handling of the additional pilot signals may be advantageous.Hereby, the transmitting apparatus 62 and/or the receiving apparatus 69may determine or obtain corresponding information from a third entity ifthe frequency notch includes pilot locations that are needed for thechannel estimation. If this is true, the additional pilot signalsimmediately adjacent to the frequency notch are inserted on thetransmitter side and evaluated on the receiver side as described. If nopilot locations are affected by the frequency notch, no additionalpilots are inserted. Also, it is possible to dynamically notch parts ofthe frequency band which should not be used for signal transmission onlyfor specific times and locations. Typically, the transmitting apparatus62 may signal the presence of the notches to the receiving apparatus 69and/or vice versa. This approach is advantageous in bi-directionalsystems, such as digital cable systems and the like, in which asignalling of data between the transmitter and the receiver is enabled.The present invention is also suitable for a semi-permanent approach ina situation where the transmitting apparatus 62 and the receivingapparatus 69 operate in an environment where the notching of certainfrequency ranges is always necessary, wherein the notching and theadditional pilot signals or the pilot pattern shifting is prestored inthe transmitter and the receiver when initializing the system.

1. A transmitting apparatus for transmitting signals in a multi-carriersystem, in which pilot signals and data mapped on frequency carriers aretransmitted in a transmission bandwidth, wherein a part of saidtransmission bandwidth is not used to transmit signals, comprising: apilot signal mapper configured to map pilot signals onto selectedfrequency carriers according to a pilot pattern that is adapted for achannel estimation in a corresponding receiving apparatus, said pilotpattern including additional pilot signals on frequency carriersadjacent to said part of said transmission bandwidth that is not used totransmit signals and enabling a channel estimation for frequencycarriers next to said part of said transmission bandwidth that is notused to transmit signals.
 2. The transmitting apparatus according toclaim 1, wherein frequency carriers with said pilot signals are embeddedwithin frequency carriers with said data.
 3. The transmitting apparatusaccording to claim 2, wherein, when said pilot pattern has adistribution of said pilot signals in a time dimension, said pilotsignal mapper is configured to map said additional pilot signals onfrequency carriers adjacent to said part of said transmission bandwidththat is not used to transmit signals with a time distributioncorresponding to a time distribution of the pilot pattern.
 4. Thetransmitting apparatus according to claim 2, wherein said pilot signalmapper is configured to map said additional pilot signals on everyfrequency carrier adjacent to said part of said transmission bandwidththat is not used to transmit signals in the time dimension.
 5. Thetransmitting apparatus according to claim 1, wherein frequency carrierswith said pilot signals are arranged in at least one training pattern,and frequency carriers with said data are arranged in a data pattern,wherein said pilot signal mapper is configured to use a pilot patternwith additional pilot signals on frequency carriers for said datapattern adjacent to said part of said transmission bandwidth that is notused to transmit signals.
 6. The transmitting apparatus according toclaim 5, wherein, when said pilot pattern has a distribution of saidpilot signals in a time dimension, said pilot signal mapper isconfigured to map said additional pilot signals on frequency carriers ofsaid data pattern adjacent to said part of said transmission bandwidththat is not used to transmit signals with a time distributioncorresponding to a time distribution of the pilot pattern.
 7. Thetransmitting apparatus according to claim 5, wherein said pilot signalmapper is configured to map said additional pilot signals on everyfrequency carrier for said data pattern adjacent to said part of saidtransmission bandwidth that is not used to transmit signals in the timedimension.
 8. The transmitting apparatus according to claim 1, whereinsaid pilot signal mapper is configured to change said pilot patterndepending on a presence of a part of said transmission bandwidth that isnot used to transmit signals.
 9. The transmitting apparatus according toclaim 1, wherein frequency carriers with said pilot signals are embeddedwithin frequency carriers with said data, wherein said pilot signalmapper is configured to shift the pilot pattern in a frequency dimensionso that a channel estimation for frequency carriers next to said part ofsaid transmission bandwidth that is not used to transmit signals ispossible.
 10. A transmitting method for transmitting signals in a multicarrier system, in which pilot signals and data mapped on frequencycarriers are transmitted in a transmission bandwidth, wherein a part ofsaid transmission bandwidth is not used to transmit signals, the methodcomprising: mapping pilot signals onto selected frequency carriersaccording to a pilot pattern that is adapted for a channel estimation ona receiving side, said pilot pattern including additional pilot signalson frequency carriers adjacent to said part of said transmissionbandwidth that is not used to transmit signals and enabling a channelestimation for frequency carriers next to said part of said transmissionbandwidth.
 11. A receiving apparatus for receiving signals in amulti-carrier system, in which pilot signals and data mapped onfrequency carriers are transmitted in a transmission bandwidth, whereina part of said transmission bandwidth is not used to transmit signals,comprising: a channel estimator configured to perform a channelestimation for received signals on the basis of pilot signals arrangedin a pilot pattern, said pilot pattern including additional pilotsignals on frequency carriers adjacent to said part of said transmissionbandwidth that is not used to transmit signals and enabling a channelestimation for frequency carriers next to said part of said transmissionbandwidth that is not used to transmit signals.
 12. The receivingapparatus according to claim 11, wherein frequency carriers with saidpilot signals are embedded within frequency carriers with said data. 13.The receiving apparatus according to claim 12, wherein, when said pilotpattern has a distribution of said pilot signals in a time dimension,said channel estimator is configured to perform a channel estimation onthe basis of said pilot pattern including additional pilot signalsmapped on frequency carriers adjacent to said part of said transmissionbandwidth that is not used to transmit signals with a time distributioncorresponding to a time distribution of the pilot pattern.
 14. Thereceiving apparatus according to claim 12, wherein said channelestimator is configured to perform a channel estimation on the basis ofsaid pilot pattern including said additional pilot signals mapped onevery frequency carrier adjacent to said part of said transmissionbandwidth that is not used to transmit signals in a time dimension. 15.The receiving apparatus according to claim 11, wherein frequencycarriers with said pilot signals are arranged in at least one trainingpattern and frequency carriers with said data are arranged in a datapattern, wherein said channel estimator is configured to perform achannel estimation on the basis of said pilot pattern includingadditional pilot signals mapped on frequency carriers for said datasymbols adjacent to said part of said transmission bandwidth that is notused to transmit signals.
 16. The receiving apparatus according to claim15, wherein, when said pilot pattern has a distribution of said pilotsignals in a time dimension, said channel estimator is configured toperform a channel estimation on the basis of said pilot patternincluding said additional pilot signals mapped on frequency carriers ofsaid data pattern adjacent to said part of said transmission bandwidththat is not used to transmit signals with a time distributioncorresponding to a time distribution of the pilot pattern.
 17. Thereceiving apparatus according to claim 15, wherein said channelestimator is configured to perform a channel estimation on the basis ofsaid pilot pattern including said additional pilot signals mapped onevery frequency carrier for said data symbols adjacent to said part ofsaid transmission bandwidth that is not used to transmit signals in atime dimension.
 18. The receiving apparatus according to claim 11,wherein said channel estimator is configured to perform a channelestimation on the basis of pilot patterns that are changed depending ona presence of a part of said transmission bandwidth that is not used totransmit signals.
 19. The receiving apparatus according to claim 11,wherein frequency carriers with said pilot signals are embedded withinfrequency carriers with said data, wherein said channel estimator isconfigured to perform a channel estimation on the basis of a changedpilot pattern shifted in a frequency dimension so that a channelestimation for frequency carriers next to said part of said transmissionbandwidth that is not used to transmit signals is possible.
 20. Areceiving method for receiving signals in a multi-carrier system, inwhich pilot signals and data mapped on frequency carriers aretransmitted in a transmission bandwidth, wherein a part of saidtransmission bandwidth is not used to transmit signals, the methodcomprising: performing a channel estimation on received signals on thebasis of pilot signals arranged in a pilot pattern, said pilot patternincluding additional pilot signals on frequency carriers adjacent tosaid part of said transmission bandwidth that is not used to transmitsignals and enabling a channel estimation for frequency carriers next tosaid part of said transmission bandwidth that is not used to transmitsignals.
 21. A system, comprising: a transmitting apparatus according toclaim 1; and a receiving apparatus configured to receive signalstransmitted from said transmitting apparatus, the receiving apparatusincluding a channel estimator configured to perform a channel estimationfor the received signals on the basis of pilot signals arranged in apilot pattern, said pilot pattern including additional pilot signals onfrequency carriers adjacent to said part of said transmission bandwidththat is not used to transmit signals and enabling a channel estimationfor frequency carriers next to said part of said transmission bandwidththat is not used to transmit signals.
 22. A method for transmitting andreceiving signals in a multi-carrier system, in which pilot signals anddata mapped on frequency carriers are transmitted in a transmissionbandwidth, wherein a part of said transmission bandwidth is not used totransmit signals, the method comprising: mapping pilot signals ontoselected frequency carriers according to a pilot pattern that is adaptedfor a channel estimation on a receiving side, said pilot patternenabling a channel estimation for frequency carriers next to said partof said transmission bandwidth; transmitting said signals; and receivingsaid signals according to the method of claim 20.